The present invention relates generally to electrical power instrumentation and more particularly relates to a watt/watthour transducer for metering the rate and quantity of electrical energy transmission.
The conventional watt/watthour metering apparatus which is related to the present invention typically has voltage and current input scaling means for providing a signal proportional to power line voltage and a signal proportional to power line current. These signals are applied to a multiplier circuit which provides an output signal having a value substantially proportional to the power or rate of energy flow through the power line.
Some systems, such as that shown in U.S. Pat. No. 3,794,917 have a multiplier which includes a pulse width modulating circuit. In such a circuit, one of the input signals or a signal proportional thereto is modulated by the other input signal or a signal proportional thereto. The output of such a pulse width modulator arrangement consists of a series of pulses having a height proportional to the instantaneous value of either the current or voltage in the power line and having a pulse width proportional to the instantaneous value of the other.
The average value of this series of pulses is proportional to the average instantaneous power of the power line. Consequently, the output of the modulator may be filtered and amplified to provide an output signal indicative of instantaneous power (actually power averaged over a very small time interval dependent upon the response time of the system which is typically less than a few seconds).
In addition, a signal proportional to the modulator output signal or the modulated output signal itself may be integrated with respect to time to provide a signal corresponding to watthours of energy through the power line over a given time interval. The present invention relates to improvements in watt/watthour metering apparatus.
It is conventional in a circuit of the above type to utilize voltage signals and voltage devices. This means that the information contained at various points in the circuitry is related to the voltage between particular nodes rather than to the current flow.
It is typical in the integrating section of the watt/watthour meter to provide circuitry which not only integrates the watt related signal with respect to time but quantizes the signal into a series of countable output pulses each pulse indicating a unit of power line energy. Such pulses may conveniently be counted or accumulated in a suitable register or counting device to indicate the total energy through the power line.
One problem with such conventional circuits, which depend upon one or more voltage signals, is that the offset voltages of typical op-amps, drift and the voltage drops across various circuit elements cause considerable error. This error is a particular problem in equipment for which error tolerances must be kept within a percentage of reading rather than within a percentage of full scale because a small offset error may be a major portion of a small reading. In fact, because of offset error one leader in the field has declared a charge compensation integrator to be unsuitable for watthour circuitry.
It is therefore an object of the invention to eliminate such voltage related error and to greatly improve the percent of reading accuracy of a watt/watthour transducer.
It is conventional in the integrating portion of the conventional watthour apparatus that the integrating circuit always operates in the same direction and that other circuitry is provided for detecting the direction of power flow. The latter circuit is sometimes referred to as power in-power out detector. The terms "in" and "out" are used equivalently to "import" and "export" or "forward" and "reverse" when referring to the direction of power flow. Circuitry is conventionally provided which reverses the signal input to the integrating circuit when the power flow direction reverses so that the integrator can always run in the same direction. The output is then steered to the appropriate counter. The power inpower out detector controls the direction of the input signal and the steering of the output. However, the signal reversing circuits introduce error because their two states are not identical.
Therefore, it is an object of the present invention to provide an integrating circuit which can integrate in either a forward or reverse direction; that is, which can integrate a watt related signal which is proportional in both amplitude and direction to the power flow through the power line being monitored.
A charge compensation integrator circuit is an integrator which has previously been used for other applications. It is also known as a "charge balance", "charge dispensing" or "charge conservation" integrator. In a conventional charge compensation integrator, an integrator circuit is charged by the signal to be integrated. Thus, the input signal may for example insert charge into the capacitor of the integrator. When the total charge reaches a selected level a precise charge quantum is removed or dumped from the integrator by other circuitry. Consequently, the number of times the precise quantum of charge is removed is an indication of total energy through the power line. A pulse may therefore be derived each time a quantum of charge is removed and these pulses can be counted or accumulated to indicate the total time integral of the watt input signal.
However, in conventional circuits, integration is done in only one direction. This means that if the watt signal places charge into the integrator the other circuitry takes charge out. For a particular conventional circuit these roles can not be reversed. The watt signal can not also take charge out with the other circuitry putting in a precise quantum of charge to produce oppositely directed pulses which can be counted.
Therefore another object of the invention is to provide a charge compensation integrator which is capable of inserting or removing equally precise countable quanta of charge into or out of a charge compensation integrator, so that the integrator can integrate in either direction.
The need for such an integrator requires the existence of a source which can provide current pulses having a very precise magnitude and width. Furthermore, a bi-directional source is required which can provide such current pulses in either direction.
Conventionally, a current source which can be sunk to virtual ground is obtained by tying a zener voltage reference diode either to the +V or -V terminal of a power supply and connecting its other end through a resistor to ground. A current source is then connected to a reference from that diode. Although this works well for providing a single current source, it is difficult to use such a zener diode for referencing other current sources and for referencing other cirucits within a watt/watthour transducer.
Accordingly, it is another object of the present invention to provide a circuit capable of producing a current pulse of precise magnitude.
Still another object of the present invention is to provide a bipolar current source which can provide current pulses in either of two directions with the pulses in either direction having identical magnitudes determined by the same single reference voltage and the same single resistance.
It is another object of the present invention to provide such a current source which utilizes a single zener-derived reference voltage which is referenced to ground so that it can be used for referencing a variety of other circuits in the watt/watthour transducer.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating the preferred embodiments of the invention.